The present invention relates to a method for washing a polycarbonate/organic solvent solution, precisely to a method for efficiently washing a polycarbonate/organic solvent solution included impurities after polymerization to purify it with an aqueous washing solution in a process of producing polycarbonate through interfacial polymerization (with phosgene), and to an apparatus for washing it.
As having good mechanical properties including impact resistance, and having good heat resistance and high transparency, polycarbonates are used in various fields of electric and electronic appliances, machine parts, automobile parts, disk plates for information-recording media such as optical memory discs, etc. In general, polycarbonates are produced through interfacial polymerization (in solvent) of an aromatic dihydroxy compound such as bisphenol A with phosgene.
For producing them, also known is a method of melt polycondensation of diphenyl carbonate with an aromatic dihydroxy compound such as bisphenol A. However, the melt polycondensation method is not always satisfactory, since the reaction temperature for it is high and the reaction time is long, and since the impurities such as the catalyst residue in the polymers produced therein often have negative influences on the hue, the heat resistance, the steam resistance and the weather resistance of the polymers produced. For these reasons, therefore, interfacial polymerization is generally used for practical production of polycarbonates in view of the quality of the polymers produced.
In the process of producing polycarbonates through interfacial polymerization, the impurities that are side-reaction product as a result of the polymerization reaction, and other impurities, especially inorganic salts such as alkali halides, alkali hydroxides, alkali carbonates and their ions, as well as non-reacted bisphenol A, monophenolic compounds used as a molecular weight-controlling agent, amines used as a catalyst and others remain in the polymers produced. Even though a little, such impurities will lower the tinting strength, the heat stability, the steam resistance and the craze resistance of the polycarbonates.
To remove these impurities from them, the polymer solution is washed and purified in an apparatus equipped with a stirring tank, a multi-stage extraction column, an orifice column, a line mixer or the like, in which is used an aqueous washing solution. The washing method for the polymers may be grouped into two, a batch system and a continuous system. In any of these washing systems, however, the efficiency of washing a polycarbonate/organic solvent solution is low when the viscosity of the solution is high, and, as a result, the impurities could not be efficiently removed from the solution and it is difficult to increase the purity of the solution.
Accordingly, for example, proposed is a method of using a two-stage stirring tank, in which a polymer solution is first washed while it is in the form of a water-in-oil dispersed phase, then this is transformed into an oil-in-water dispersed phase easy of phase separation, and thereafter it is led into a static separation tank (Japanese Patent Publication No. 38967/1984). Also proposed is a method of first processing a polymer solution in an orifice column to have a high pressure loss, and thereafter leading it into a large-size static separation tank (Japanese Patent Publication No. 37032/1982). However, these washing methods require such large-size units, complicated operations and a long washing time, and therefore the productivity in the methods is low. In addition, the washing efficiency in the methods is not always satisfactory.
To solve the problems, some other washing methods for polycarbonate/organic solvent solutions are proposed. For example,  less than 1 greater than  Japanese Patent Publication No. 53793/1994 discloses a method that comprises mixing a crude polycarbonate/organic solvent solution and an aqueous washing solution into a mixture having an aqueous phase content of from 5 to 30% by volume, then stirring the mixture in a line mixer being driven at a power of at least 0.1 kW/m3/hr for the unit flow rate of the mixture to thereby form a water-in-oil dispersed phase, and thereafter centrifuging the resulting mixture. Japanese Patent Publication No. 39523/1994 also discloses a method that comprises mixing a polycarbonate/organic solvent solution and an aqueous washing solution to form a water-in-oil dispersed phase, then processing it to thereby make the liquid drops in the dispersed phase have a particle size of at most 100 xcexcm, and thereafter centrifuging the resulting mixture having the thus-controlled dispersed phase.
 less than 2 greater than  Japanese Patent Laid-Open No. 309940/1995 also discloses a method for washing an impurities-containing polycarbonate solution with an aqueous washing solution, in which the polycarbonate solution is first mixed with the aqueous washing solution in a mixer to form an oil-in-water emulsion mixture, and thereafter the resulting mixture is filtered through a filter layer to thereby separate it into a phase of the polycarbonate solution and a phase of the aqueous washing solution.
In the former  less than 1 greater than  of these improved washing methods, a water-in-oil dispersed phase is formed in the step of mixing a polycarbonate/organic solvent solution with an aqueous washing solution; while in the latter  less than 2 greater than , an oil-in-water dispersed phase is formed in that step. In this point,  less than 1 greater than  and  less than 2 greater than  quite differ from each other. It is believed that these washing methods are both good.
However, the latter method  less than 2 greater than  of forming the oil-in-water dispersed phase requires a large amount of the aqueous washing solution relative to the polycarbonate solution. Concretely, they say that the amount of the aqueous washing solution they need is from 0.7 to 5 times by volume of the polycarbonate solution, and actually in the Examples demonstrated by them, it is from 1.2 to 1.5 times by volume. Therefore, in the method, the volume of the mixture is large, and the method requires large-size and high-performance units for mixing the solutions and for separating the resulting mixture. In addition, the amount of the wash waste in the method greatly increases. The waste water contains the solvent for polycarbonates, which is most typically dichloromethane. The cost for recovering the solvent is high, and this will be a serious problem with the method.
On the other hand, the former method  less than 1 greater than  of forming the water-in-oil dispersed phase is characterized in that the amount of the aqueous washing solution needed therein may be extremely small, as compared with that needed in the washing method  less than 2 greater than . Concretely, the amount of the aqueous washing solution in  less than 1 greater than  may be from 5 to 30% by volume of the polycarbonate/organic solvent solution, and, in fact, it is from 10 to 25% by volume as in the Examples demonstrated by the inventors of the method  less than 1 greater than .
Accordingly, in the methods of washing polycarbonate/organic solvent solutions, any of the polycarbonate solution and the washing solution may form a dispersed phase, depending on the condition of the dispersion (emulsion) of the mixture of the two solutions, for washing the polycarbonate solution. In these methods, however, when the mixture forms an oil-in-water phase and when the amount of the aqueous washing solution is small, the efficiency of washing the polymer solution lowers to a great extent. Therefore, as in the method  less than 2 greater than , the phase ratio of the aqueous washing solution in the mixture having an oil-in-water phase must be high. Needless-to-say, also in the water-in-oil dispersed phase in the method  less than 1 greater than , the phase ratio of the aqueous washing solution may be high for attaining higher washing efficiency.
However, the emulsion condition of the mixture dispersion composed of a polycarbonate/organic solvent solution and an aqueous washing solution is significantly influenced by the change of the properties of the two solutions that constitute the mixture dispersion. Specifically, the properties of the polycarbonate/organic solvent solution that vary depending on the terminal structure of the polycarbonate, the molecular weight thereof, the viscosity of the polymer solution, the change of the resin concentration in the solution and the impurity content of the polymer solution, and also the properties of the aqueous washing solution that vary depending on the pH of the solution and the impurity content thereof have significant influences on the formation of the dispersed phase of the mixture dispersion of the two solutions.
Accordingly, even though the ratio of the aqueous washing solution to be fed to the polycarbonate/organic solvent solution is controlled to be all the time constant, the condition of the dispersed phase of the mixture of the two solutions could not be kept all the time constant, and it involves a problem of phase transition between a water-in-oil dispersed phase and an oil-in-water dispersed phase. In fact, therefore, the blend ratio of the aqueous washing solution in the mixture dispersion must be relatively lowered, or that is, the washing condition to be actually employed inevitably sacrifices a part of the washing efficiency for keeping the water-in-oil dispersed phase of good washability.
The present invention is to provide an economical method of washing a polycarbonate/organic solvent solution with an aqueous washing solution, in which the amount of the aqueous washing solution to be used can be reduced while stabilizing the washing efficiency at a high level, in particular, to provide such a washing method for a polycarbonate/organic solvent solution capable of being effected even in a small-size apparatus and in a continuous process line, and to provide a washing apparatus for the method.
We, the present inventors have assiduously investigated the above-mentioned problems with the washing method for polycarbonate/organic solvent solutions, and, as a result, have found that the dispersion (emulsion) condition and also the phase transition of the mixture of a polycarbonate solution and an aqueous washing solution have significant influences on the washability of the polycarbonate solution, and that the phase transition of the mixture dispersion is accompanied by the viscosity change thereof. In addition, we have further found that, when the blend ratio of the aqueous washing solution in the mixture is suitably controlled based on the viscosity change of the mixture, then the polymer solution can be washed stably and its washing efficiency can be kept at a high level. On the basis of these findings, we have completed the present invention.
Specifically, the invention provides the following:
1. A method for washing an impurities-containing polycarbonate/organic solvent solution with an aqueous washing solution, which comprises mixing and dispersing the polycarbonate/aqueous solution with an aqueous washing solution, monitoring the viscosity of the resulting mixture dispersion, confirming the phase condition of the mixture dispersion from the viscosity change thereof, and controlling the blend ratio of the aqueous dispersion solution to the polycarbonate/organic solvent solution on the basis of the relation between the phase condition and the viscosity of the mixture dispersion of the two solutions.
2. The method for washing the polycarbonate/organic solvent solution of above 1, wherein the phase condition of the mixture dispersion of the polycarbonate/organic solvent solution and the aqueous washing solution is so controlled that the mixture dispersion forms essentially a water-in-oil dispersed phase.
3. The method for washing the polycarbonate/organic solvent solution of above 1, wherein the blend ratio of the aqueous washing solution to the polycarbonate/organic solvent solution is automatically controlled.
3. The method for washing the polycarbonate/organic solvent solution of above 1, wherein the polycarbonate content of the impurities-containing polycarbonate/organic solvent solution falls between 5 and 30% by weight, and the aqueous phase fraction of the mixture dispersion of the polycarbonate/organic solvent solution and the aqueous washing solution falls between 10 and 40% by volume.
4. The method for washing the polycarbonate/organic solvent solution of above 1, wherein the viscosity of the mixture dispersion of the polycarbonate/organic solvent solution and the aqueous washing solution is monitored with an oscillational viscometer.
5. The method for washing the polycarbonate/organic solvent solution of above 1, wherein the mixture dispersion of the polycarbonate/organic solvent solution and the aqueous washing solution is formed in a line mixer.
6. The method for washing the polycarbonate/organic solvent solution of above 5, wherein the stirring power in the line mixer is at least 0.1 kW/m3/hr for the unit flow rate therein.
7. The method for washing the polycarbonate/organic solvent solution of above 1, wherein a purified polycarbonate/organic solvent solution is separated from the mixture dispersion of the polycarbonate/organic solvent solution and the aqueous washing solution mixed in the controlled blend ratio.
8. An apparatus for washing the polycarbonate/organic solvent solution, which comprises a unit for mixing and dispersing an impurities-containing polycarbonate/organic solvent solution and an aqueous washing solution, a unit for monitoring the viscosity of the resulting mixture dispersion, a unit for confirming the phase condition of the mixture dispersion from the thus-monitored viscosity change of the mixture dispersion, and controlling the blend ratio of the two solutions on the basis of the thus-confirmed phase condition, and a unit for separating a purified polycarbonate/organic solvent solution from the mixture dispersion.
9. The apparatus for washing the polycarbonate/organic solvent solution of above 8, wherein the unit for monitoring the viscosity of the mixture dispersion is an oscillational viscometer.
10. The apparatus for washing the polycarbonate/organic solvent solution of above 8, wherein the unit for mixing and dispersing the impurities-containing polycarbonate/organic solvent solution and the aqueous washing solution is a line mixer, and the unit for separating the purified polycarbonate/organic solvent solution from the mixture dispersion is a centrifugal separator.